This invention relates in general to filter holders and filter systems, and more specifically to systems utilizing disposable tubular filtration elements with an inside-to-outside forced fluid flow.
In most filtering processes, it is necessary to support the filter medium, to direct an unfiltered fluid flow toward one surface of the filter medium and to direct the filtered fluid away from another surface of the medium. The need for support is particularly acute when the filter medium is a thin layer of material such as a membrane filter, or when the unfiltered fluid is under an applied pressure to promote or speed the filtering process. To direct the fluid flow to and from the filter medium, it is known to mount a filter medium within a housing so that the filter separates two fluid zones or reservoir chambers, one through which the unfiltered fluid passes and the other through which the filtrate passes. The construction of the housing and the nature of the reservoir chambers and mounting arrangements have taken a wide variety of forms depending on such factors as the shape and characteristics of the filter medium, and degree of the applied pressure, the desired flow rate, characteristics of the fluid, and convenience in periodically replacing the filter. To support fragile filter media, particularly under high applied pressures, it is also known to place a mechanically strong support layer against the downstream side of the filter medium. Common supports include wire meshes and apertured layers of various structural materials.
To enhance the mechanical strength of the filter medium and to provide a relatively large filter surface in a compact form, it is common to wrap a sheet of filter medium into a tubular form. Filter elements of this design, and associated supports layers and holders, are common in the field of reverse osmosis filtering such as water desalinization and fluid concentration where the applied pressure is typically in the range of hundreds of psi. U.S. Pat. No. 3,578,175 to Manjikian is representative of such reverse osmosis filters and employs a hollow perforated core of a structural material which is wrapped with the filter membrane and other supportive or protective layers. The inner core provides downstream support for the filter layer with an outside-to-inside fluid flow direction. U.S. Pat. No. 3,744,771 to Manjikian discloses a suitable filtration system for mounting these tubular reverse osmosis filters with the outside-to-inside flow direction. Another type of tubular reverse osmosis filter is described in U.S. Pat. No. 3,457,170 to Havens which utilizes an inside-to-outside flow direction with a resin-impregnated outer support layer enclosing the filter media.
None of these conventional filtration systems, however, are adapted to operate with a disposable tubular filtration element of the type disclosed in a commonly assigned, co-pending U.S. application Ser. No. 565,094, filed of even date herewith by Merrill et al, the disclosure of which is hereby incorporated by reference. As there described, these filtration elements have a laminate construction in which an intermediate layer is formed from a microporous filtration material that removes all particulate matter and bacteria having dimensions in excess of a predetermined size from a fluid flowing through the material. A significant characteristic of the filtration material is that it exhibits a growth in volume of approximately 6% when it is wet (commonly termed "wet growth"). Another significant characteristic of the filtration material is that it is highly fragile and must be extremely well supported when in use, particularly when there is an applied pressure differential across the material. Filtration systems utilizing such elements must therefore provide good downstream support while accommodating the wet growth. In addition, the wet growth must be accommodated in a manner which reduces or eliminates any flexing or bending of the filtration medium since the mechanical stress of these movements can readily weaken or rupture the medium and thereby destroy its effectiveness as an absolute filter.
Downstream support and wet growth accommodation are especially significant when the filtration system is used to remove bacteria from the fluid and thereby "cold" sterilize it. In such applications, there are significant cost and convenience advantages in the capability of in-line sterilization of the filtration system itself, which involves the passage through the system of steam at a temperature in excess of 121.degree. C at pressures of 15 psi. Conventional filter systems are inadequate in that an outside-to-inside pressurized fluid flow wrinkles, and therefore weakens or ruptures, the wet growth expanded filtration material. Also, the differential expansion of dissimilar materials, when steam sterilized sometimes results in fracture of the filter material. Of the few inside-to-outside flow filter systems known in the art, none are adapted to simultaneously accommodate an outward wet growth expansion while at the same time providing the necessary downstream support. Further, conventional filter systems are in general not adapted to operation under aseptic conditions commonly associated with microporous filtration elements, and, in particular, these systems are not designed to allow a fast and reliable replacement of spent filtration elements with fresh filtration elements or a rapid and convenient disassembly and reassembly of the filtration system for periodic, thorough cleaning and autoclaving.
It is therefore a principal object of our invention to provide a filtration system that utilizes a disposable tubular filtration element with an inside-to-outside forced fluid flow and which provides good downstream support for the filtration element while accommodating for its wet growth without subjecting it to mechanical stresses which would weaken or rupture the filtration medium.
Another object of the invention is to provide a filtration system for the removal of bacteria from a fluid which is in-line steam sterilizable.
Still another object of the invention is to provide a filtration system in which it is convenient to load and unload the filtration elements under aseptic conditions.
Still another object of the invention is to provide a rugged and compact filtration system which is readily installed in a production line, readily disassembled and reassembled once installed, and has a relatively low cost of manufacture.
Other and further objects of our invention will in part be obvious and will in part appear hereinafter.